Dell MX9116n EMC SmartFabric OS10 User Guide - Page 493

Bidirectional Forwarding Detection The Bidirectional Forwarding Detection (BFD) protocol rapidly detects communication failures between two adjacent routers. BFD replaces link-state detection mechanisms in existing routing protocols. It also provides a failure detection solution for links with no routing protocols. BFD provides forwarding-path failure detection in milliseconds instead of seconds. Because BFD is independent of routing protocols, it provides consistent network failure detection. BFD eliminates multiple protocol-dependent timers and methods. Networks converge is faster because BFD triggers link-state changes in the routing protocol sooner and more consistently. BFD is a simple hello mechanism. Two neighboring routers running BFD establish a session using a three-way handshake. After the session is established, the routers exchange periodic control packets at sub-second intervals. If a router does not receive a hello packet within the specified time, routing protocols are notified that the forwarding path is down. In addition, BFD sends a control packet when there is a state change or change in a session parameter. These control packets are sent without regard to transmit and receive intervals in a routing protocol. BFD is an independent and generic protocol, which all media, topologies, and routing protocols can support using any encapsulation. OS10 implements BFD at Layer 3 (L3) and with User Datagram Protocol (UDP) encapsulation. BFD is supported on static and dynamic routing protocols, such as VRRP, OSPF, OSPFv3, IS-IS, and BGP. The system displays BFD state change notifications. NOTE: BFD is only supported for the border gateway protocol (BGP). BFD session states To establish a BFD session between two routers, enable BFD on both sides of the link. BFD routers can operate in both active and passive roles. • The active router starts the BFD session. Both routers can be active in the same session. • The passive router does not start a session. It only responds to a request for session initialization from the active router. A BFD session can occur in Asynchronous and Demand modes. However, OS10 BFD supports only Asynchronous mode. • In Asynchronous mode, both systems send periodic control messages at a specified interval to indicate that their session status is Up. • In Demand mode, if one router requests Demand mode, the other router stops sending periodic control packets; it only sends a response to status inquiries from the Demand mode initiator. Either peer router, but not both, can request Demand mode at any time. A BFD session can have four states: Administratively Down, Down, Init, and Up. The default BFD session state is Down. • Administratively Down - The local BFD router does not participate in the session. • Down - The remote BFD router is not sending control packets or does not send them within the detection time for the session. • Init - The local BFD outer is communicating to the remote router in the session. • Up - Both BFD routers are sending control packets. A BFD session's state changes to Down if: • A control packet is not received within the detection time. • Demand mode is active and a control packet is not received in response to a poll packet. BFD session state changes example The session state on a router changes according to the status notification it receives from the peer router. For example, if the current session state is Down and the router receives a Down status notification from the remote router, the session state on the local router changes to Init. Layer 3 493